Multiscale Characterization to Examine the Effects of Aggregate Properties on Aggregate-Paste Interphase in Cement Concrete Mixtures

This study aims to test the effects of aggregate properties on the features of aggregate-paste interphase in portland cement concrete mixtures. The microstructural, chemical, and nanomechanical properties of the interphase region, formed because of the interaction of two commonly used chemically-distinctive aggregates (i.e., limestone as a calcite aggregate and quartzite as a siliceous aggregate) with ordinary portland cement paste, were examined through multiscale measurements. More specifically, the microstructural, chemical, and nanomechanical properties at the interphase zone were characterized using laser scanning microscopy, scanning electron microscopy coupled with energy dispersive spectroscopy, and nanoindentation. Furthermore, a three-point bending test was used to evaluate the bond between the aggregate and paste on single edge notched beam specimen where a thin aggregate sheet was inserted. A coupled microstructural, mechanical, and chemical examination can provide integrated characterization of an interphase region formed by different aggregate properties. It was found that the thickness of interfacial debonding between aggregate and paste is more dominantly influenced by moisture absorption capacity, while the surface chemistry of the aggregates did not significantly affect the characteristics of the interphase. It was also observed that when there is a good bonding between aggregate and paste, ample calcium silicate hydrate (C-S-H) gel is formed close to the aggregate surface, which is demonstrated by similar Ca/Si ratios between the interphase region and adjacent paste region. On the other hand, poor interphase and resulting interfacial debonding was associated with more involvement of CH crystals at the interphase region, which was observed by greater Ca/Si ratios.